This invention relates broadly to electrochemical process for the removal of metal-ion contaminants from aqueous media, and more particularly to improvements in a process wherein such contaminants are reduced and precipitated at an iron anode. This invention was made in the course of, or under, a contract with the United States Energy Research and Development Administration.
In a conventional process for the removal of metal-ion radical contaminants from an aqueous medium, the contaminant-containing medium is passed continuously through an electrochemical cell on a once-through basis. The cell contains a sacrificial iron anode for effecting reduction and precipitation of a mixed-hydroxide precipitate containing the contaminants. Where, for example, the contaminant is hexavalent chromium in the form of chromate or dichromate, a mixture of chromium hydroxide and iron hydroxide is precipitated at the anode, whereas gaseous hydrogen is evolved at the cathode. The mixed precipitate typically consists of very small particles which are suspended in the aqueous medium outflow from the cell. (For further details on the chromate-removal mechanism, see "Chromate Removal from Cooling Tower Blowdown by Reaction with Electrochemically Generated Ferrous Hydroxide," Journal of Environmental Science and Technology, Vol. 7, No. 4, April, 1973).
Various techniques have been used to recover the above-mentioned suspended precipitate from the aqueous medium withdrawn from the electrochemical cell. Hitherto, the most satisfactory techniques have been sedimentation or filtration, but neither of these techniques has proved to be efficient. Sedimentation involves undesirably long settling times, filtration is not well suited to the removal of such small particles unless the filters are replaced frequently. Thus, separation of the precipitate has posed a continuing problem and has restricted application of the overall process.